Context note: "in-universe", the english text-to-speech words would be in a mixture of Inklish and Octolang, and are presented in english here both for the convenience of real-world listeners and because my experiments with Nonsense Words™ for Splatoon tracks in the past were unsatisfactory. After Deca gained more fame and acclaim, this early version of the song was re-released as part of a compilation that brought together different mixes of the track, and subsequently found its way into an edition of the popular rhythm game Squid Beats. This song, in-universe, is one of their very first productions done on the surface- an early version of a song of theirs that would later become one of their big hits and lead anthems. They also, as a consequence of Skint being the one to teach them Inklish, have a very slang heavy and informal dialect (comparatively, when they do speak Octolang, their syntax is very polite and proper they actually speak an antiquated form of the language that is usually reserved solely for speaking with ones superiors, politicians, leaders etc). Deca subsequently began to produce music how they wanted to, without expectation or any purpose beyond making the listener feel good their tracks, which fell within the Hard Trance/Hard Dance/Tech Dance genres usually, began to circulate through the underground rave scene in Inkopolis with increasing interest and acclaim, allowing Deca both to eventually settle into their own residence (and make a comfortable living as a producer and DJ under the name Rising Octave, even breaking into the mainstream rave scene) and, finally, come out of their shell and discover who they really were- which, as it turns out, is an INSANELY upbeat party animal who almost never sleeps, can drink almost anyone in the city under the table, and is constantly "on" and exciteable. They struck up a friendship, travelled together, Skint would eventually get Deca set up at a shelter for a short while so they could find their feet and, crucially, allowed Deca to use some of the music equipment he had on hand (both the stuff he was repairing, and stuff he owned).
They were homeless and destitute and survived largely due to the kindness of other homeless individuals- this includes a couple of my other characters, Skint and Brassic, who bumped into Deca around about the same time they were going through their own personal tragedy. They were present for the confrontation between the New Squidbeak Splatoon and DJ Octavio that closes the events of Splatoon 1, and thus heard the Calamari Inkantation as they were already unhappy and unsatisfied with their position, the song- and seeing its subsequent positive effect on the crowd- gave them the final push they needed to abandon their post and head for the surface. They loved music, but hated what they made and what it was used for- their loyalty to the Octarian forces was shaky basically from the start. As quick a character summary as I can manage here (plus some specific context to the song) Deca is a gender neutral Octoling, a musical prodigy who was conscripted into DJ Octavio's Music Propeganda program (alongside the canonical Octoling group Turqouise October) at a very young age.